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Definition of evolution

L.A. Moran lamoran at gpu.utcs.utoronto.ca
Fri Nov 1 12:16:12 EST 1991

This thread is becoming very complex because several different issues are 
being debated at the same time. The main theme is the definition of evolution.
I have defended a common textbook definition which is;

     "Biological evolution is a change in the frequency of alleles
      in a population."

Arlin Stoltzfus objects to definitions like this on several grounds and I
hope to address all of his points in subsequent postings. However, I would
first like to clear up an important misunderstanding. Arlin says,

     "If you start to think about evolution in an open-minded way, you 
      should be able to think of more phenomena that cannot be explained 
      by allele replacements. This is especially true if you are not one
      of those molecular biology types who can only think of DNA when 
      asked to think about evolution."

He also said,

     "Larry and many others define evolution in terms of successive 
      allele replacements, and Larry goes a bit further in suggesting
      that a single shift in the frequencies of alleles at a locus
      constitutes evolution. I do not disagree that a succession of 
      allele replacements constitutes evolution, but rather that 
      evolution comprises more than allele replacements."

I don't believe that I have EVER said that evolution can be defined solely
in terms of allele REPLACEMENTS and I don't know of any evolutionary
biologist who makes such a claim. I always understood the common definition
to include gene duplications, recombinations, deletions, etc. Arlin seems
to recognize that many of us take this broad view of "a change in the 
frequency of alleles" but he persists in putting the words "allele
replacements" in my mouth. He says,

     "With regard to the phenomena of non-allelic genomic change,
      Larry's response was that these could be considered to be the 
      results of allele replacements. Since Larry and I seem to make 
      contradictory statements about the same thing, I suspect that 
      we will have to go into a few detailed examples in order to 
      determine whether or not molecular drive (for instance) can be 
      modelled by allelic changes. However, it seems clear that Larry's 
      view of allele replacements is a bit too broad at present. In 
      response to my contention that the symbiotic association of an 
      urkaryote and a bacterial proto-mitochondrion was a significant
      evolutionary event but not in any rigorous sense an allele 
      replacement, Larry again suggested that this did not 'clearly fall
      outside of the [allele-replacements] definition.'"

In an attempt to improve the definition of evolution to cover all examples
of changes in the genome, Arlin makes the following suggestion;

     "Larry and other readers interested in salvaging genetic definitions
      of evolution would do well to make the following corrections:

      First, avoid limiting the circumscribed phenomena to allele
      replacements. It's much better to talk about "statistical 
      fluctuations in the genetic composition of populations" (Sewall
      Wright's definition), since this will include allele replacements,
      molecular drive, etc., which are legitimate evolutionary phenomena."

I don't think that the textbook definition of evolution that I quoted ever
intended to limit evolution to allele REPLACEMENTS but nevertheless Arlin's
point is well taken. Since there is some confusion on this issue it would
be preferable to choose a definition that was more general. I found the
following examples in introductory biology textbooks;

     "Evolution [is] a change in gene frequencies."
                            Wessels and Hopson, BIOLOGY, 1988; Random House
     "Evolution: genetic change in a population of organisms over time."
                       Raven and Johnson, BIOLOGY, 1989; Times Mirror/Mosby
     "Evolution [is a] change in the genetic makeup of a population."
                       Keeton and Gould, BIOLOGICAL SCIENCE, 1986; W.W.Norton

Taking these ideas into account I propose the following definition;

      Biological evolution is the process of change in the genetic
      makeup of a population.

This definition is necessary and sufficient (IMHO).

Laurence A. Moran (Larry)
Dept. of Biochemistry
University of Toronto

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